Our long term objective is to understand biochemical events during infection of cells by RNA tumor viruses. These events are believed to lead to the establishment in the infected cell of a provirus, the carrier of hereditary information for producing virus. Many experiments suggest that the provirus is a DNA copy of the viral RNA, and it is presumed to be made during infection by the RNA-dependent DNA polymerase carried in virions of RNA tumor viruses. We are exploring three approaches in studying these biochemical events: (1) the study of the DNA homologous to synthetic viral DNA in uninfected and infected cells, characterization of any difference between them, and study of the development of the difference during establishment of infection; (2) search for factors in infected or uninfected cells which affect the reaction rate of the viral RNA- dependent DNA polymerase or the size, secondary structure, or complexity of its product; (3) study of virus production by hybrid somatic cell lines.